The following Document 1, for example, discloses one communications scheme which simultaneously sends a radio frequency band (RF) signal generated by up-converting an intermediate frequency band (IF) signal, and a local oscillation (LO) signal used for the up-conversion. The reception side of this communications scheme performs demodulation using the received LO signal and RF signal. Accordingly, a receiver can eliminate an expensive local oscillator mounted therein, thus providing a low-cost communications system.
Document 1: JP-A-2001-53640
The following Document 2 describes that in such a communications scheme, the RF signal must be equal in power to the LO signal under the conditions of constant transmission power, in order to optimize the carrier to noise ratio (C/N) index on the receiver side. For this reason, the IF input power must be set at a point at which the RF output power matches the LO output power, so that the amount of back-off from P1 dB (1 dB gain compression point) in a transmitter is determined by this matching point.
Document 2: Yozo Shoji et al, “60 Ghz Band 64 QAM/OFDM Terrestrial Digital Broadcasting Signal Transmission by Using Millimeter-Wave Self-Heterodyne System,” IEEE TRANSACTIONS ON BROADCASTING, VOL. 47, No. 3, September 2001, pp. 218-227
Here, the back-off amount will be described with reference to FIG. 1. Referring to FIG. 1, there is shown a relationship between the output power of the LO signal (labeled LO) and the output power of the RF signal (labeled RF) to PIF which is the IF input power, where the amount of power from P1 dB, which is a 1 dB compression point of the RF signal, in a decreasing direction is called the back-off amount. A reduction in the back-off amount brings the RF signal into a nonlinear region, causing a problem of distortions in the output. Therefore, an optimal value exists for the back-off amount, however, as mentioned above, the IF input power must be set at point A at which the RF output power matches the LO output power, so that the back-off amount from P1 dB in the transmitter is determined by this matching point.
Consider herein that a conventional image rejection mixer is used in the communications scheme described in the aforementioned Patent Document 1. FIG. 2 is a diagram illustrating the configuration of a conventional image signal rejection mixer. Power splitter 101 splits an LO signal generated from an LO signal oscillator into two components which are supplied to mixer 103 and mixer 104. Power splitter 102 splits an IF signal into two components which are supplied to mixer 103 and mixer 104. Mixers 103 and 104 each generate an RF signal from the LO signal and IF signal supplied thereto, and supply the RF signal and LO signal to power combiner 105. Power combiner 105 combines the LO signal and RF signal delivered from mixer 103 and the LO signal and RF signal delivered from mixer 104, and delivers the combined signal with a suppression image.
Here, power splitter 101 splits the LO signal into two components which are equal in amplitude and have phase difference α. Power splitter 102 splits the IF signal into two components which are equal in amplitude and have phase difference β. Power combiner 105 combines the powers with an equal amplitude and a phase difference γ. In this event, α, β, γ are set such that the phase relationship satisfies:α+β+γ=2nπ  (1) andα−β+γ=(2n+1)π (n is an integer)  (2)in order to maximize an image signal rejection ratio. In this event, the LO power has a value determined by:α+γ=(2n+½)π  (3)
Incidentally, when the quality of communication varies following variations in the communication environment, good communications cannot be made unless the modulation scheme is changed to be suitable to a particular communication environment (quality of communication). Then, it is known that an optimal back-off amount exists for P1 dB of the transmitter output in a modulation scheme, and for this reason, if the back-off amount is changed in line with variations in the communication environment, a problem arises in that the modulation scheme cannot be changed such that it is suitable to the communication environment.
When communications are made as described in Patent Document 1 using a conventional image rejection mixer, the LO power has a value determined by a phase:α+γ=(2n+½)π
When communications are made to result in optimal reception C/N, the back-off amount is determined to be at IF power at which the LO output power is equal to the RF output power, resulting in a failure in communications made at an back-off amount suitable to a communication environment.